Since a fuel cell using a solid polymer electrolyte membrane exhibits a low working temperature of 100° C. or lower and a high energy density, it is expected to put it into practical use in power sources for electric vehicles, simplified auxiliary power sources for electric/electronic devices, domestic fixed power sources, and the like. In the field of solid polymer electrolyte membrane-type fuel cell, there are included important elemental technologies on a solid polymer electrolyte membrane, a platinum-based catalyst, a gas-diffusion electrode, a conjugate of the solid polymer electrolyte membrane with the gas diffusion electrode, and the like. Among these, it is one of the most important technologies to develop a solid polymer electrolyte membrane having good properties as a fuel cell.
In the solid polymer electrolyte membrane-type fuel cell, a gas diffusion electrode is combined with both sides of a solid polymer electrolyte membrane and the solid polymer electrolyte membrane and the gas diffusion electrode substantially form an integral structure. Therefore, the solid polymer electrolyte membrane acts as an electrolyte for conducting protons and also plays a role as a diaphragm for preventing direct mixing of hydrogen or methanol as a fuel with an oxidizing agent even under elevated pressure. As such a solid polymer electrolyte membrane, it is required to have a large migration rate of protons and a high ion-exchange capacity and to have a constant and high water retentivity for maintaining a low electrical resistance, as an electrolyte. On the other hand, in view of the role as a diaphragm, it is also required to have a large mechanical strength, an excellent dimensional stability, an excellent chemical stability against a long-term use, no excessive permeability to hydrogen gas or methanol as a fuel and oxygen gas as an oxidizing agent, and the like.
In an early solid polymer electrolyte membrane-type fuel cell, an ion-exchange membrane of a hydrocarbon resin produced by copolymerization of styrene with divinylbenzene was used as an electrolyte membrane. However, this type of electrolyte membrane is low in durability and hence poor in practicality. Thereafter, a fluororesin-based perfluorosulfonic acid membrane “Nafion (registered trademark of Du Pont)” developed by Du Pont has been commonly used.
Conventional fluororesin-based electrolyte membranes such as “Nafion” are excellent in chemical durability and stability. However, in the case that they are employed in a direct methanol fuel cell (DMFC) where methanol is used as a fuel, they have a problem of occurrence of a crossover phenomenon that methanol passes through the electrolyte membranes, resulting in a decreased output.
Furthermore, since the fluororesin-based electrolyte membranes are produced with starting from monomer synthesis, there is a problem that production thereof requires many steps and a high cost, which is a large obstacle for their practical use.
Therefore, it has been attempted to develop a low cost electrolyte membrane which may be substituted for the above “Nafion” or the like, and there have been proposed processes for producing solid polymer electrolyte membranes by introducing a sulfonic acid group into fluororesin-based membranes (e.g., see, Patent Documents 1 to 3).
Patent Document 1: JP-A-2001-348439
Patent Document 2: JP-A-2002-313364
Patent Document 3: JP-A-2003-82129